Horizontal shear instabilities in rotating stellar radiation zones: I. Inflectional and inertial instabilities and the effects of thermal diffusion

TL;DR

This paper explores how horizontal shear instabilities in stellar radiative zones are affected by stratification, rotation, and thermal diffusion, identifying two main types of instabilities and their behaviors under various conditions.

Contribution

It provides a detailed numerical and analytical study of inflectional and inertial instabilities in rotating stellar zones, highlighting the effects of thermal diffusion and stratification.

Findings

Inflectional instability is destabilized by stratification and stabilized by thermal diffusion.

Inertial instability's growth rate peaks at a specific value of the Coriolis parameter.

Both instabilities exhibit self-similarity when scaled with PeN^2.

Abstract

Rotational mixing, the key process in stellar evolution, transports angular momentum and chemical elements in stellar radiative zones. In the past two decades, an emphasis has been placed on the turbulent transport induced by the vertical shear instability. However, instabilities arising from horizontal shear and the strength of the anisotropic turbulent transport that they may trigger remain relatively unexplored. This paper investigates the combined effects of stable stratification, rotation, and thermal diffusion on the horizontal shear instabilities in the context of stellar radiative zones. The eigenvalue problem describing linear instabilities of a flow with a hyperbolic-tangent horizontal shear profile was solved numerically for a wide range of parameters. As a first step, we consider a polar $f$-plane where the gravity and rotation vector are aligned. Two types of instabilities are identified: the inflectional and inertial instabilities. The inflectional instability that arises from the inflection point is the most unstable when at a zero vertical wavenumber and a finite wavenumber in the streamwise direction along the imposed-flow direction. The three-dimensional inflectional instability is destabilized by stratification, while it is stabilized by thermal diffusion. The inertial instability is rotationally driven, and a WKBJ analysis reveals that its growth rate reaches the maximum $\sqrt{f(1-f)}$ in the inviscid limit as the vertical wavenumber goes to infinity, where $f$ is the dimensionless Coriolis parameter. The inertial instability for a finite vertical wavenumber is stabilized as the stratification increases, whereas it is destabilized by the thermal diffusion. Furthermore, we found a self-similarity in both the inflectional and inertial instabilities based on the rescaled parameter $PeN^2$ with the P\'{e}clet number $Pe$ and the Brunt-V\"{a}is\"{a}l\"{a} frequency $N$.